1. Field of the Invention
This invention relates generally to a signal playback device of the type in which a light beam scans a record carrier for reproducing a signal recorded thereon, and more particularly is directed to improvements in the optical system of such a signal play-back device by which a predetermined focussing of the scanning light beam at a surface of the record carrier on which the signal is recorded is automatically maintained in spite of vibrations or other movements of the record carrier in directions normal to such surface.
2. Description of the Prior Art
It has been proposed to optically record a signal, such as, a video signal, on a flat record carrier, for example, in the form of a disc, and to optically read the recorded signal by means of a light beam scanning a surface of the carrier on which the signal is recorded. In optically reading the recorded signal, as aforesaid, it is necessary that the light beam be maintained in a properly focussed condition in respect to the surface of the record carrier having the signal recorded thereon. However, when a rotated disc is employed as the record carrier, as is usually the case, some vibration or movement of the record disk in directions normal to its opposed surfaces is practically unavoidable. As a result of such vibtation or movement, the distance between the surface of the disc havimg the signal recorded thereon and a lens provided for focussing the scanning light beam in respect to such surface is varied and undesirably disturbs the proper focussing of the light beam in respect to the surface of the record disc.
In order to maintain the proper focus of the light beam in respect to the surface of the record disk, it has been proposed to provide the optical signal play-back device with a servo system which detects the instantaneous position of the surface of the rotating record disk and, in response to any detected movement of such surface in the directions normal thereto, effects relative movement of the focussing lens and the plane of rotation of the rotating disk for maintaining the predetermined distance therebetween required for proper focussing of the light beam. However, such servo system is relatively complicated and costly and, furthermore, it is difficult for the servo system to accurately compensate for the rapid variation in the distance between the focussing lens and the record disc surface by reason of the relatively large masses that need to be moved for effecting the necessary compensation.
In view of the above, it has been proposed to provide an optical signal play-back device with an auto-focussing optical system for the light beam which scans the record carrier surface. For example, in U.S. Pat. Nos. 3,932,701 and No. 3,959,581, and in the present inventor's U.S. patent application Ser. No. 658,999, filed Feb. 18, 1976, and having a common assignee herewith, an auto-focussing optical system is employed with a record carrier, for example, a record disc, provided with first and second oppositely facing reflective surfaces and having the signal recorded as variations in at least the second surface. Such auto-focussing optical system may comprise a first lens directing a light beam from a source thereof against the first record carrier surface for reflection by the latter and passage back through the first lens for focussing by the latter at an image point remote from the first record carrier surface, and a second lens for effecting a predetermined focussing of such image point of the light beam on the second record carrier surface for scanning the recorded signal variations. The light beam reflected from the second record carrier surface, and having variations in its beam energy corresponding to the recorded signal variations, is passed back through the second lens toward a light detector for providing an output corresponding to the variations in the reflected beam energy. Further, in the foregoing auto-focussing optical system, the first and second lenses thereof have respective magnification factors selected so that the product thereof is equal to 1/.sqroot.2 for maintaining the predetermined focussing of the image point of the light beam in respect to the second record carrier surface irrespective of movements of the latter toward and away from the second lens.
Although the auto-focussing optical system, particularly as disclosed in detail in U.S. patent application Ser. No. 658,999, identified more fully above, is effective to maintain the proper focus of the light beam in respect to the surface of the record carrier having the signal recorded thereon, a number of full-reflecting and semi-reflecting mirrors have to be included in the optical system at various angles for establishing the path of the light beam between the source thereof and the light detector. As a practical matter, it is difficult and costly, in terms of the labor involved in assembling the optical system, to accurately locate the various mirrors in the optical signal playback device so as to ensure that the optical axis of the light beam, in passing through the second lens of the auto-focussing optical system prior to scanning the second surface of the record carrier, will exactly coincide with the optical axis of such second lens. In the event that the axis of the light beam, in thus passing through the second lens, deviates from the optical axis of the second lens, the direction of the light beam, when passing back through the second lens toward the light detector, will vary substantially in response to changes in the distance between the second lens and the second surface of the record carrier. The amount of light received by the light detector will be varied as a function of such substantial changes in the direction of the light beam directed toward the detector. Thus, the output of the light detector will be influenced by the movement of the record carrier toward and away from the second lens of the auto-focussing optical system and will no longer correspond accurately to the recorded signal variations.